The present invention relates to a nozzle for a liquid cooled plasma torch, a nozzle cap for a liquid cooled plasma torch and a plasma torch head with same.
A plasma is an electrically conductive gas thermally heated to a high temperature and consisting of positive and negative ions, electrons and excited and neutral atoms and molecules.
Different gases are used as plasma gas, for example the single-atom argon and/or the two-atom gases hydrogen, nitrogen, oxygen, and air. These gases ionise and dissociate through the energy of an arc. The arc constricted through a nozzle is described as a plasma beam.
The parameters of a plasma beam can be greatly influenced by the form of the nozzle and electrode. Such parameters of the plasma beam can, for example, include the beam diameter, temperature, energy density and the flow speed of the gas.
In plasma cutting, for example, plasma is constricted through a nozzle which can be gas cooled or water cooled. Energy densities of up to 2×106 W/cm2 can thereby be reached. Temperatures of up to 30,000° C. arise in the plasma beam, which realize, in association with the high flow speed of the gas, very high material cutting speeds.
Plasma torches can be operated directly or indirectly. In a direct mode of operation, current flows from a current source via the electrode of a plasma torch. The plasma beam produced by means of an arc and constricted through the nozzle directly via the work piece back to the current source. Electrically conductive materials can be cut with such direct mode of operation.
In an indirect mode of operation, current flows from the current source via the electrode of a plasma torch, the plasma beam, produced by means of an arc and constricted through a nozzle, and the nozzle back to the current source. The nozzle is thereby more greatly loaded than during direct plasma cutting, as it does not only constrict the plasma beam but also realizes the starting point of the arc. With such indirect mode of operation, both electrically conductive and non-electrically conductive materials can be cut.
Due to high thermal load, nozzles are generally made from a metal material, preferably from copper due to its high electrical conductivity and heat conductivity. The same applies to the electrode holders, which are also frequently made from silver. The main components of a plasma torch include a plasma torch head, a nozzle cap, a plasma gas guiding part, a nozzle, a nozzle holder, an electrode receiving element, an electrode holder with electrode insert and, in modern plasma torches, a nozzle protection cap holder and a nozzle protection cap. The electrode holder fixes a sharp electrode insert made of tungsten, which is suited for the use of non-oxidizing gases such as plasma gas, for example an argon-hydrogen mixture. A flat electrode, of which the electrode insert is made, for example, of hafnium, is also suited for the use of oxidizing gases such as plasma gas, for example air or oxygen. In order to achieve a longer lifespan for the nozzle, the latter is cooled with a liquid such as water. The coolant is supplied via a water supply element to the nozzle and carried away from the nozzle by a water return element and thereby flows through a coolant chamber, which is delimited by the nozzle and the nozzle cap.
Former East Germany document DD 36014 B1 describes a nozzle. This consists of a material with good conductivity, for example copper, and has a geometric form assigned to the respective plasma torch type, for example a conically formed discharge chamber with a cylindrical nozzle outlet. The outer form of the nozzle is formed as a cone, whereby a virtually equal wall thickness is achieved, and whereby such dimensions allow that good stability of the nozzle and good head conduction to the coolant. The nozzle is located in a nozzle holder. The nozzle holder consists of corrosion resistant material, for example brass, and has internally a centring receiving element for the nozzle and a groove for a sealing rubber, which seals the discharge chamber against the coolant. Furthermore, bores offset by 180° are disposed in the nozzle holder for the coolant supply and return. On the outer diameter of the nozzle holder there is a groove for a rubber o-ring for sealing the coolant chamber in relation to the atmosphere and also a thread and a centring receiving element for a nozzle cap. The nozzle cap, made of a corrosion resistant material such as brass, is formed at an acute angle and has a wall thickness usefully dimensioned to facilitate removal of radiation heat to the coolant. The smallest inner diameter is provided with an o-ring. Water is used as a coolant in the simplest case. This arrangement is intended to facilitate simple manufacture of the nozzles with sparing use of materials and rapid exchange of the nozzles as well as allowing, through acute angle construction, a pivoting of the plasma torch in relation to the work piece to allow for inclined cuts.
German document DE-OS 1 565 638 describes a plasma torch, preferably for plasma fusion cutting of work pieces and for preparation of welding edges. The narrow form of the torch head is achieved through the use of a particularly acute-angled cutting nozzle, of which the inner and outer angles are equal to each other and also equal to the inner and outer angle of the nozzle cap. A coolant chamber is formed between the nozzle cap and the cutting nozzle, in which coolant chamber the nozzle cap is provided with a collar, which seals metallically with the cutting nozzle, so that an even annular gap is thereby formed as a coolant chamber. The supply and removal of the coolant, generally water, is realized through two slots in the nozzle holder, which are arranged offset in relation to each other by 180°.
German document DE 25 25 939 describes a plasma arc torch, particularly for cutting or welding, wherein the electrode holder and the nozzle body form an exchangeable unit. The outer coolant supply is formed essentially through a clamping cap enclosing the nozzle body. The coolant flows via channels into an annular space, which is formed by the nozzle body and the clamping cap.
German document DE 692 33 071 T2 relates to a plasma arc cutting device. An embodiment of a nozzle is described therein for a plasma arc cutting torch, which nozzle is formed from a conductive material and comprises an outlet opening for a plasma gas beam and a hollow body section. Said body section is formed so that it has a generally conical, thin-walled configuration, which is inclined towards the outlet opening, and has an enlarged head section, which is formed integrally with the body section. The head section is thereby solid with the exception of a central channel, which is aligned with the outlet opening and has a generally conical outer surface, which is also inclined towards the outlet opening and has a diameter adjacent to that of the adjacent body section which exceeds the diameter of the body section, in order to form an undercut recess. The plasma arc cutting device has a secondary gas cap. A water cooled cap is arranged between the nozzle and the secondary gas cap in order to form a water cooled chamber for the outer surface of the nozzle for highly effective cooling. The nozzle is characterised by a large head, which surrounds an outlet opening for the plasma beam, and a sharp undercut or a recess to a conical body. This nozzle construction encourages the cooling of the nozzle.
In the plasma torches described above the coolant is supplied through a water supply channel to the nozzle and carried away from the nozzle by a water removal channel. These channels are mostly offset by 180° relative to each other and the coolant is intended to flow around the nozzle as evenly as possible on the way from the supply to the removal channel. Nonetheless, overheating in proximity to the nozzle channel is ascertained again and again.
Former East Germany document DD 83890 B1 describes another coolant guide for a torch, preferably a plasma torch, in particular for plasma welding, plasma cutting, plasma fusion and plasma spraying purposes, which withstands high thermal loads of the nozzle and the cathode. A coolant guide ring, which can be easily inserted into the nozzle holding part and easily removed from it, is provided for the cooling of the nozzle. Said coolant guide ring has, for the purpose of limitation of the coolant guide to a thin layer of maximum 3 mm in thickness, along the outer nozzle wall, a surrounding groove. Running into this surrounding groove are multiple cooling lines, preferably two to four in number, which are arranged in a star form radially thereto and symmetrically to the nozzle axis and in a star form in relation thereto at an angle of between 0 and 90°, such that the cooling lines are respectively adjacent two coolant outflows and each coolant outflow is adjacent to two coolant inflows.
However, such arrangement has the disadvantage that greater resources are necessary for the cooling through the use of an additional component, the coolant guide ring. In addition, such arrangement requires a larger construction.